"I treated it with H2O2 and its been fine ever since, but I'm curious if anyone has any ideas what caused it?"

I can not explain the magic of hydrogen peroxide either. Keep running it with your system at all times. There is hygrozyme too, I have used it and it works very well!!!!

I think what your looking at is the bacteria that forms soap scum. I think your water is to blame.

Why is the water to blame?

True the problem is in the water. But why do you think that's where it originated from? The post didn't even say what the water source was. Contaminates could enter the water from many sources. If so, then changing the water source wont do a thing to help.

I also have a question that may be somewhat obvious, but is the "white scum" soft like mold, or hard calcium/lime buildup?

Have your water tested to see about iron in your water. The iron can come from your water system as well. Check out your bath room for scum on the walls.

First thing on my mind is don't waist money on having a water test done. If you have many systems, but only one with the problem (as you stated), it's not the water source. Second thing on my mind is three points really. First every bathroom shower/tub/sink has scum from soap (and in my case large amounts of conditioner) buildup if you don't regularly clean it. Second is that most hard scum is calcium/lime buildup and white in color (not iron). Third is high amounts of iron will result in red rust looking stains, we have high amounts of iron in our water, and even though we have a water softener you still sometimes see the rust stains in the toilet. But for us the rust stains are more prevalent outside, where that water dosen't go through the water softener.

Bottom line is if you are using the same nutrients etc., and have many systems as you stated, you would be seeing the same problem in all of them using the same water source. I wouldn't waist money on unnecessary lab testing. If large amounts of iron were somehow feeding a microorganism (and possible), it would still be in all your systems as well.

water is reverse osmosis filtered and ionized. The tomato is a clone that was cloned all alone.Only thing I can think of is something may have happened in the cloner.

The scum is a thick, soft, congealed goo. It formed in the net pot first (hence my bad clone theory) and mostly on the bottom. It takes on the color of the nutes but seems to be white. I had a secondary infection on it again, flushed the (net)pot with running water. No sign of anything in the nutrient solution, but I changed it. New roots are growing above the scum accumulation and the H2O2 foams like crazy when it contacts it. Its only in one pot with one plant using the same water and nutrients all the others are

I am going to let you in on a pro hint, I know, I know ;)

You are for sure running into an issue with too much fertilizer, not right water temp, ect...
Well, lets face it we all can not keep up on that.

Use a bar of soap in the reservoir. It really will work. If your system is small then a small cube.

Sounds like pythium, usually caused by high res temps/lack of oxygen.

bar of soap? I've never heard of that.

do you use ivory so it will float, or should you use something that sinks?